Dr. E. Naumann—NMagnetism and Earth Structure. 537 
of Kreil’s observation, the other for the later epoch, show a most 
remarkable similarity. To the east, the isogonic lines are parallel 
to the Carpathians, and they turn round near Kronstadt and 
Fogarasch, just as the Transylvanian Alps do. The centre of Tran- 
sylvania is traversed by a loop. We thus see that the isogones 
present a thoroughly natural form, and that their characteristic 
features remain unaltered, notwithstanding the secular changes of 
magnetism. This is certainly a very important result. 
A considerable number of other illustrations might be quoted, but 
I am afraid of digressing too far, and must refer those who take 
a special interest in the matter to the above-mentioned paper, in 
which a discussion of all the magnetic surveys hitherto accomplished 
will be found. Many pages of it are devoted to the results of the 
_ Magnetic and Geological Survey of Japan, carried out under my 
direction during the years 1880-85. Those results are of special 
importance to our subject, and I may therefore be allowed to submit 
some observations which have come under my own experience. 
There is a most remarkable correspondence between the lines of 
equal declination and the principal lines of geological structure. 
In general the magnetic lines exhibit striking and unexpected 
irregularities, and these irregularities are found to be most intimately 
connected with the abnormal curvatures of the folds. The serious 
discussion which followed the reading of a paper of mine before the 
Seismological Society of Japan in 1882! showed how far these 
irregularities were unexpected. For my own part I was convinced 
from the very beginning of the Geological Survey, at a time when 
the magnetic data were still scanty, that there must be some connection 
between the phenomena caused by terrestrial magnetic force and the 
internal condition of the earth’s crust or of the earth itself. With 
this point in view the magnetic investigations were commenced. 
In a comparatively short time the general magnetic survey, com- 
prising no less than 200 complete observations at a like number 
of stations, was accomplished by Mr. Sekino, one of my former 
topographical assistants. The results are very satisfactory. It will 
be observed that the course of magnetic lines are influenced by the 
great transverse cleft called Fossa Magna (see Plate XIX.). This is 
a great depression, cleft, or fissure, running from the Pacific to 
the Japan Sea, in which a number of volcanoes have sprung up. 
Fujinoyame, for instance, the highest mountain of the Japanese 
Archipelago, is situated in this cross fissure. The Japanese chain 
consists of a long series of folds, running as a rule in the same 
direction as the island chain itself, but in the neighbourhood of the 
Fossa Magna these folds (which were probably raised by a force 
acting from the continent towards the Pacific) turn back as if they 
had been stopped by the great wedge of eruptive rocks lying below 
the Fossa Magna. In Mr. Sekino’s Magnetic Map (Plates XVIII. and 
XTX.) the 5° isogone will be seen to cireumscribe the Fossa Magna, for 
in that vicinity the curve forms a great wave whose crest is upturned 
' Notes on Secular Changes of Magnetic Declination in Japan, vol. v. of the 
Trans, Seism. Soc. of Japan. 
